I do not think they care at all about what GregTech does. If someone dislikes what GregTech does to another mod, they can modify the config files as GregTech always has config options for changes to other mods. It's a waste of their time.
Can never be too sure. They might get pissed if he used his mod to cause their own to have different results. I know I would personally. If I were to ever write a mod it would specifically malfunction with GT.